During the operation of a diesel engine, air is first drawn into a combustion chamber and then compressed until it reaches a temperature above the auto-ignition temperature of a diesel fuel which is then injected into the combustion chamber under high pressure in the form of a spray or mist (i.e. as fine droplets). Under the conditions of high temperature and pressure, the diesel fuel ignites and combusts explosively to drive a piston movable within the combustion chamber. Unlike a gasoline engine, a diesel engine does not use an electrical spark to initiate fuel ignition. Therefore, a diesel fuel should have a high flash point and a low auto-ignition temperature. The flash point is the lowest temperature at which the fuel would ignite in the presence of a flash source, such as an electrical spark, in the presence of air. The auto-ignition temperature is a temperature at which the fuel automatically ignites in the presence of air without a source of ignition.
For diesel fuels, the so-called “cetane number” is a characteristic of diesel fuel quality. A diesel fuel of high cetane number combusts cleanly, having a short ignition delay leading to more time to combust, more uniform combustion and cleaner exhaust emissions. Such characteristics also lead to better fuel utilization by minimizing soot formation and increasing power output. The cetane number of a fuel can be determined by measuring the ignition delay of the fuel, i.e. the time period between the start of fuel injection and the start of combustion (ignition). Fuels with good ignition quality have short ignition delays. The ignition delay is affected by many factors such as the engine configuration (e.g. fuel injection orifice diameter) and engine operating conditions (gas cylinder ambient temperature and pressures) and fuel components, including the presence of fuel additives. Generally, diesel engines run well with diesel fuels having a cetane number from 40 to 55, and there is no performance or emission advantage when the cetane number is raised past approximately 55, although premium diesel fuels sold in Europe may have cetane numbers as high as 60 and all diesel fuels sold there are currently required to have a minimum cetane number of 51.
In North America, most States and Provinces adopt ASTM D975 as their diesel fuel standard and the minimum cetane number is set at 40, with typical values in the 42-45 range. Premium diesels may or may not have higher cetane numbers, depending on the supplier, and it is expected that minimum cetane number requirements may increase in the future in line with the European standards.
Premium diesel fuels often use additives to improve their cetane number ratings and their fuel lubricity, detergents to clean fuel injectors and minimize carbon deposits, water dispersants to avoid ignition problems, and other additives depending on geographical and seasonal needs.
Diesel fuels are generally distilled from crude oils. However, crude oils entering the market are tending to become heavier as sources of lighter crudes become depleted and the diesel fuel fraction (middle distillates) produced from such crude oils by distillation tend to have a reduced content of aliphatic hydrocarbons and an increased content of aromatic hydrocarbons. Unfortunately, aromatics tend to reduce cetane numbers of middle distillates and result in diesel fuels exhibiting longer ignition delays, as well as exhibiting poorer combustion characteristics (such as generation of soot).
Many raw hydrocarbon diesel fuels have cetane numbers less than 40 and the current practice for increasing the cetane number of such fuels is to add a small amount of a cetane number improvder, generally an alkyl nitrate, such as EHN (2-ethyl hexyl nitrate). EHN itself has a cetane number of about 350 and can thus be used in very small amounts to increase the cetane number of a raw fuel. The additive also has good performance and cost-effectiveness. When EHN is added to diesel fuel in an amount of about 0.1 vol. %, it produces a cetane number increase of about 1 to 2. When the EHN concentration is increased further, the cetane number increases accordingly, but less effectively. The increase in cetane number that can be achieved by the addition of EHN reaches a plateau at about 0.2 to 0.3 vol. %, at which level the cetane number increase reaches about 8 above the starting value of the fuel. Beyond such concentrations, there is no further beneficial effect of EHN addition.
There is therefore a need for ways of improving the cetane number ratings of diesel fuels, especially ways that can achieve cetane number increases greater than single digits.
U.S. Pat. No. 6,692,634, which issued to Yakovlevich et al. on Feb. 17, 2004, discloses a process for increasing the cetane and octane numbers of hydrocarbon fuel involving the production of a turbulent biphasic mixture of fuel and an ozone-containing gas in a flow-through chamber followed by the collection in a container having a stable pressure level. The patent states that the process involves oxonolysis and hydrogenation and that it involves the use of an electrohydrodynamic converter, which implies some form of electrochemical conversion. The patentee maintains that it is possible by this process to increase the octane or cetane number of the fuel by 3 to 5 units.
U.S. Pat. No. 5,762,655, which issued to Horst Kief on Jun. 9, 1998, discloses a method of producing an improved hydrocarbon fuel for internal combustion engines and turbines. The fuel is subjected to ozonization either forming a current of the fuel and bubbling an ozone-oxygen mixture in countercurrent through the fuel, or by enriching the fuel with oxygen and subjecting the enriched fuel to ultraviolet radiation. The patent makes no reference to cetane numbers.
US patent application US 2002/0079272 A1, which was published on Jun. 27, 2002 naming Jeffrey Sherman as inventor, discloses a method of improving the quality of diesel fuel in which an oxidizing gas, preferably ozone, is formed into sub-micron sized bubbles which are dispersed into the fuel. It is said that sulfur is removed from the fuel and the cetane rating thereof is increased.
US patent application US 2006/0211906 A1, which was published on Sep. 21, 2006 naming Ilya Zborovsky as inventor, discloses a method of purifying a liquid medium involving oxidizing the liquid medium with an oxidant using a sorbent material having an impregnated particulate catalyst. The oxidant may be air, ozone, hydrogen peroxide or other gas known for oxidizing techniques. The oxides of impurities are absorbed by the sorbent and are then separated and removed by washing the sorbent with a polar solvent, which may include alcohols. It is mentioned in paragraph [0098] that the liquid medium may be a hydrocarbon and that aromatic compounds may be removed by the process, thereby increasing the cetane rating of diesel fuel after hydrotreating because of the increased proportion of aliphatic hydrocarbons.
PCT patent publication WO 01/32809 A1, which was published on May 10, 2001 naming Raphael Caers, et al., as inventors, discloses the oxidation of distillate fuel in the presence of titanium silicate catalyst to produce hydroxyl and carbonyl groups bonded to paraffinic carbon atoms of diesel fuel molecules to provide at least 0.1 wt. % oxygen in the fuel. While the use of ozone is mentioned in the patent, it is clear that the oxidation is carried out using hydrogen peroxide in the presence of a solvent used to enable the distillate fuel and hydrogen peroxide to interact and come into contact with the catalyst. A large amount of the solvent is required (e.g. 70 vol. %, as shown in Table 1). Cetane numbers above 50 or 52 are desired.
PCT patent publication WO 2005/052098 A1, which was published on Jun. 9, 2005 naming Graham Ketley, et al., as inventors, discloses a process to improve the cetane number and emissions characteristics of distillate feedstocks by increasing the oxygen content of the feedstock. The feedstock is contacted with an oxygen-containing gas in the presence of an oxidation catalyst on a basic support. The publication teaches against the addition of expensive chemical oxidizing agents such as organic peroxides, ozone or hydrogen peroxide and uses oxygen instead under the action of a catalyst.
PCT patent publication WO 2007/07531 A2, which was published on Jul. 5, 2007 naming Thomas Palmer, et al., as inventors, discloses ring-opening of naphthalenic and aromatic rings in hydrocarbon streams derived from crude oil using an oxidation catalyst in the presence of oxygen. Thus, aromatics are transformed to aliphatics, although there is no reference to any enhancement of cetane numbers. The method involves severe operating conditions (high temperature and pressure) in the presence of an oil-soluble metal catalyst.
Canadian patent 1,287,007, which issued on Jul. 30, 1991 to James Kittrell, et al., discloses a process of upgrading diesel oil by contacting it with an oxidant selected from nitrogenous oxidizing agents and ozone and then contacting the oil with an extracting solvent. The cetane number of the fuel is said to increase by at least 5 units. Methanol and ethanol are said to be unsuitable alcohols for use as solvents.
U.S. Pat. No. 6,673,236 B2, which issued on Jan. 6, 2004 to Maria Stanciulescu et al., discloses a method of producing hydrocarbon fuels with ultra-low levels of sulfur. The method involves catalytic oxidation of sulfurous compounds within the fuel, followed by the extraction of the oxidized compounds using a polar solvent. Ethanol is used during the oxidation, and it is said that the oxidant may be hydrogen peroxide, ozone, oxygen or air, but only hydrogen peroxide is exemplified. There is no mention of improvement of cetane numbers.